A digital broadcasting system executes entire processes based upon digital signals, and therefore can realize clear images and high sound quality by subdividing low-to-high tone sound signals. Accordingly, the application of the digital broadcasting system is getting spread rapidly and widely through various media such as terrestrial, satellite and cable TVs.
A test may be executed on the function and performance of a DTV receiver, which receives from the DTV broadcasting system, by using a signal from the broadcasting system. However, although an operational test may be executed based upon the signal from an actual broadcasting system, the DTV broadcasting system does not provide any signal by which a special function of the receiver can be tested or the performance of the receiver can be maintained steady. As a consequence, a developer or manufacturer of DTV receivers typically establishes an imaginary broadcasting system in order to test the function and performance of a DTV receiver via the imaginary broadcasting system.
FIG. 1 schematically illustrates a concept of a DTV test via an imaginary broadcasting system. A stream generator 101 generates an audio/video test stream to be used in a test on the function or performance of a DTV receiver, a channel encoder 102 executes channel-encoding to the test stream, a channel up-converter 103 modulates and up-coverts the channel-encoded test stream into an RF level and then outputs the up-converted test stream in the form of an RF signal, and a DTV receiver or set-top box 104 receives the RF signal. Then, a measuring instrument 105 such as an oscilloscope 105 or an HDTV/PC monitor 106 is used to execute a test to the function or performance of the DTV receiver based upon the received test stream.
The length of the test stream used in the manufacture of the DTV receiver as above must be extended infinite as in an actual broadcasting. Also very wide types of test streams are needed for the function or performance test of the DTV receiver. This in turn generates a large number of channels together with enormous data. However, since a computer storage media for storing the DTV test stream has a limited capacity, stream length is minimized and a limited length of stream is repeatedly generated. In case that the limited length of test stream is repeatedly generated, it is required to correctly synchronize the generation time of a video or audio element stream (ES) with that of a transport stream (TS). That is, when the test stream is repeatedly generated, any inconsistency between the TS and the ES may induce audio or video discontinuity, which in turn may cause a DTV receiver to be misconceived defective even though it had passed a quality test in a manufacturing site. As a consequence, means for synchronize the ES with the TS have been needed in the art.
Further, when a DTV receiver decodes a received audio/video resource constituted of a digital video stream and then outputs (i.e., outputs sound and displays a video image) through audio/video processing, any time difference may occur between audio and video signals thereby mismatching the video image and the sound. The mismatched sound and image may dissatisfy a user who watches a DTV set.
Such a process of synchronizing the audio signal with the video signal is referred to as “lip synchronization.” Occasionally, means for executing lip synchronization (e.g., hardware and/or software) may be provided inside the DTV receiver. As a well-known technique for enabling the lip synchronization, the DTV receiver uses the time information of the audio/video signals. The technique counts the time information of the audio/video signals to find whether an audio frame proceeds in advance of or behind a video frame, and if a time difference between the audio and video signals exceeding a reference time interval is found at a specific time point, discards or repeats an audio frame at that time point, thereby executing the lip synchronization.
However, even though a lip synchronization control function is mounted, a manufactured DTV receiver is subject to a test with the foregoing test equipment in order to inspect whether audio and video streams of the DTV receiver are consistent with each other. The lip synchronization is a factor to be regarded essential in a DTV system, and tested by measuring a time difference between the audio and video signals in the DTV receiver.
For example, where the time difference between the audio and video signals is in the range from 25 msec to 40 msec, that is, the audio signal proceeds in advance of the video signal within 25 msec or behind the video signal for at least 40 msec. Then, regenerated image and sound appear matching each other in this range, thereby providing more natural image and sound to the audience.
Conventionally, in order to measure a time difference between audio and video signals in a DTV receiver, a lip synchronization test is executed by using a video sequence and an audio signal, which are specifically constructed. That is, the video signal consists of a black part, a white part and a short line part. Then, a lip synchronization test is executed by inserting a signal for video signal synchronization into the short line part, and using a tone signal, which exponentially decreases at a synchronization position, as an audio test signal.
However, this method has a problem in that a video sequence and an audio sequence must have specific constructions dedicated for the lip synchronization test. Therefore, the lip synchronization test for manufactured DTV receivers require additional devices and techniques, thereby causing inconvenient test operation and poor productivity.